The present invention generally relates to a keypad status reporting system and more particularly to such a system wherein keypad status is reported over a serial bus. The present invention still more particularly relates to such a system wherein a keypad monitor provides a unique parallel multiple-bit key code indicative of a selected keypad switch and wherein a parallel to serial converter converts the parallel key code to serial bits of data and places the serial bits of data into a predetermined channel onto a multiplexed bus. The keypad status reporting system of the present invention as disclosed herein may be used to advantage in a hands-free, multiple feature telephone unit incorporated into a single integrated circuit and adapted for use in an Integrated Services Digital Network (ISDN).
Keypads and monitors associated therewith are well known in the art. Such keypads generally include an array of switches with each switch coupled between a respective different pair of first and second conductors. When a keypad switch is depressed or otherwise selected, a condition can then be sensed across the first and second conductors to which a particular switch is coupled to identify which switch is being depressed. Such condition sensing is performed by the keypad monitor.
Keypad monitors for identifying a particular keypad switch being selected have been implemented in integrated circuit form. The keypad monitors of such integrated circuits generally include a first plurality of inputs arranged to be coupled to the first keypad conductors and a second plurality of inputs arranged to be coupled to the second keypad conductors. The keypad monitors further generally include internal circuitry which scan the first and second plural inputs for detecting the closure of a keypad switch.
One dominant application for keypads and keypad monitors is in telephone equipment. Keypads are used in such applications for dialing telephone numbers or selecting various features provided by a particular piece of telephone equipment. Such features may include call transfer, call conference, hold, or multiple-line selection, for example.
Keypad monitors generally report the status of the keypad switch using a parallel multiple-bit format. This is acceptable when the keypad monitor is not integrated with other circuitry requiring integrated circuit terminal pins. However, if the keypad monitor is integrated with other circuitry requiring integrated circuit terminal pins, the cost of such an integrated circuit can be prohibitive. This is the direct result of the economic fact that in the integrated circuit industry, the cost of producing an integrated circuit is strongly dependent upon the number of terminal pins required to be incorporated into the device package. If the keypad monitor is to report the keypad switch status with a parallel six-bit key code, six terminal pins would be required for the keypad status reporting alone. Hence, in the past, the amount of other circuitry which could be integrated with a keypad monitor has been extremely limited.